Applications of pulsed electromagnetic radiation sources (primarily infrared light or visible light but also ultraviolet radiation) such as material processing or precision micro machining offer improved performance with higher fluence (energy per area) and peak intensity (energy per time per area). Such pulse parameters result in better laser micromachining performance, i.e. the material can be readily ablated with improved spatial precision and a reduction in material damage of nearby areas (so-called “cold ablation” resulting in a reduced heat-affected zone around the area of the ablated material). A further performance influencing parameter is the pulse repetition rate. In material processing applications, processing speed generally scales linearly with the repetition rate.
Therefore, radiation sources of coherent radiation combining the following properties would be desirable:                short optical pulses, in the range of picoseconds (preferably around 10-15 ps, often more than 20 ps, but even down into femtoseconds)        high pulse energies (preferably 100 microjoules or more)        high repetition rate (ideally 100 kHz or more)        preferably near diffraction limited spatial profiles (to allow for near-diffraction limited focusing to beam spot size diameters on the order of the laser wavelength i.e. one micron approximately)        if possible, readily adjustable repetition rate, over a range of pulse repletion rates, preferably from a minimum to maximum ratio of at least 4× (i.e., for example 50 kHz to 200 kHz) without having to substantially re-align or re-adjust the laser or amplifier, and while still maintaining good spatial beam properties, preferably without substantially changing key beam properties (M2, beam waist, beam divergence).        
These requirements are also beneficial if shorter wavelengths are required, because nonlinear frequency conversion efficiency increases with increasing peak intensity and with good spatial beam qualities. Shorter wavelengths (i.e. frequency doubling, tripling, quadrupling, or quintupling) decrease the potential spot size for diffraction-limited focused beams, and can improve the material processing performance for different materials, since the shorter-wavelength light has higher photon energy, resulting in different and improved ablative performance (for example in transparent materials).
Current state-of-the art diode-pumped solid-state lasers can be passively mode-locked to produce, in a simple and robust way, picosecond and femtosecond optical pulses, as, for example, disclosed in U.S. Pat. No. 5,987,049. These lasers typically produce optical pulses with low pulse energy (10-100 nJ) but at high repetition rates (e.g. 100 MHz). Also mode-locked high power lasers have been known, for example, from U.S. Pat. No. 6,834,064. However, the achievable pulse energies of these high power pulsed lasers are in the low microjoule range, which is still not enough for the initially mentioned applications.
As an alternative to high power lasers, it has been suggested to combine a mode-locked laser oscillator with an amplifier. Several embodiments of continuous-wave diode-pumped multi-pass amplifiers have been described, for example, in U.S. Pat. No. 5,546,222, U.S. Pat. No. 5,615,043, U.S. Pat. No. 5,774,489. By this technique, the average output power can be amplified considerably, even to many tens of Watts if a plurality of amplification stages are applied. However, due to the high repetition rate of the initial oscillator, the pulse energies remain below 1 μJ. An even higher average power is not desirable due to disadvantages entailed by high average power, such as potential thermal fracture, thermal lens effects, complex set up involving a multitude of pump diodes, a sophisticated heat management, high power consumption etc. Also, repetition rates exceeding some tens of MHz or hundreds of MHz may be disadvantageous since they are difficult to handle for some material processing applications. The most desirable range would be between 100 KHz and 4 to 10 MHz. However, decreasing the repetition rate of the initial oscillator would mean to increase the cavity length, which is usually not desired, since the oscillator would become physically very large or require a complex folding technique, which results in optical loss and reduced mechanical stability.
An alternative approach to decreasing the pulse repetition rate and at the same time increasing the pulse energy is “cavity dumping”. This technique comprises inserting an optical switch (typically electro-optic or acousto-optic) into the laser cavity and reducing the normal output coupling of the laser through the output-coupling mirror to as little as possible. This allows the intracavity pulse energy to increase. Occasionally, an intracavity pulse is switched out at a reduced repetition rate (typically over the range from single shot to megahertz pulse rates) but at increased pulse energy compared to the normal continuous operation of the laser. Such a cavity dumped laser can typically get ten times higher pulse energies, but the pulse energy typically becomes independent on the dumping frequency below a few MHz, so that further reductions in the repetition rate do not result in a pulse energy gain. Also, switching is inherently critical. Any misalignment of the intracavity beam with respect to the switching apparatus changes the output coupling and therefore the laser dynamics, the circulating pulse energy, etc. Further, cavity dumping perturbs the laser dynamics, since the leftover pulse has a smaller than equilibrium pulse energy, resulting in non-steady state performance. All this may lead to higher pulse-to-pulse fluctuations than in continuously mode-locked lasers, and even chaotic pulse performance.
Even if these stability problems are somehow overcome, the pulse energies are still not sufficient. It has therefore been proposed (for example Huber et al., Optics Letters 28, p. 2118 (2003)) to combine a cavity-dumped laser with a 2-pass pass continuously pumped amplifier. However, such an approach did also not result in sufficient maximum pulse energy, since the given gain material to be chosen under the given boundary conditions have a very low gain, as a consequence of which tight focusing is required. This leads to strong gain saturation (depletion) at low pulse energies. More in general, it has proven to be difficult to achieve a continuously pumped, high-gain multi-pass amplifier, and there are significant trade-offs between high total gain and high average power output.
Higher gain can be achieved in a spatial multi-pass approach if the pump power is increased. However, because of thermal problems, which are similar to the problems encountered in continuously pumped high-power lasers, this is only possible in a pulsed pump scheme at low repetition rate. An example of this state of the art may be found in Lenzner et al., Optics letters 20, p. 1397 (1995), where a TiSa mode-locked laser has been combined with a Pockels cell selecting single pulses from a 80-MHz-pulse train at a repetition rate between 1 kHz and 5 kHz and a pulsed-radiation pumped amplifier. The achieved radiation rates are not fast enough for high-speed material processing as required in industrial applications. Alternatively, systems have been proposed (for example in U.S. Pat. No. 5,812,308), where the amplifier does not have a high small-signal gain but is seeded with a high-average power oscillator and serves more as a power amplifier to increase the average power by a factor 2-4.
Yet another approach to achieve the high total gain is a regenerative amplifier, where a pulse is trapped in an amplifier cavity, and is re-circulated many times until the pulse energy has grown to where the gain material is effectively saturated. Such a regenerative amplifier has for example been disclosed in U.S. Pat. No. 4,896,119. However, since such regenerative amplifiers comprise a cavity, the misalignment sensitivity is comparably high and chaotic instabilities result in a limited range of repetition rates. Further, the optical switch has to be an electro-optic Pockels cell (acousto-optic modulators are normally not suitable due to the small beam size that would be required in them for fast switching, which would result in peak intensities, due to the high intracavity pulse energy, in the device that exceed their damage threshold). This brings about the necessity for high voltages in the system and as a consequence high-power electronics with all its disadvantages. Also, the switch is alignment-sensitive, the achievable repetition rates are limited, and it is not straightforward to change the repetition rate because the repetition rate influences the roundtrip gain and thermal lens effects, hence the optical performance.